First examples of stable carbocations are reported from 7H‐benzo[c]fluorene (2), 11H‐benzo[b]fluorene (3), 11H‐benzo[a]fluorene (4), 2‐methoxy‐ (5), 7‐methoxy‐ (6), and 9‐methoxy‐11H‐benzo[a]fluorene (7), 7H‐dibenzo[c,g]fluorene (8), 13H‐dibenzo[a,g]fluorene (9), 2‐methoxy‐13H‐dibenzo[a,g]fluorene (10) and 5,6‐dihydro‐13H‐dibenzo[a,g]fluorene (11). Charge‐delocalization modes in the resulting carbocations were derived based on experimental and/or computed (GIAO‐DFT) Δδ13C values and through the NPA‐derived changes in charges (Δq). Whereas protonation regioselectivity in the parent systems (2, 3, 4, 8, and 9) corresponds to the energetically most favored carbocations computed by DFT, selectivity in the OMe‐substituted derivatives (5, 6, 7, 10, and 11) is strongly controlled by the methoxy group. Benzofluorenes 3, 5, 6, and 7 and dibenzofluorenes 8, and 10 were nitrated under very mild conditions. Nitration selectivity in the parent systems 3 and 8 parallels those in stable‐ion protonation, whereas regioselectivity in the MeO derivatives (6, 7, and 10) corresponds more closely to relative arenium ion energies in the parent unsubstituted systems. Comparative mutagenicity assays (Ames tests) were performed on 3NO2, 5NO2, 7NO2, 8NO2, and 10NO2 relative to their precursors. Compounds 10NO2, 7NO2, and 8NO2 were found to be potent direct‐acting mutagens (with 10NO2, 8NO2 also capable of acting as potent indirect mutagens). The X‐ray structures of 5NO2 and 8NO2 were determined. The angle between the plane of the nitro group and the aromatic ring bearing the NO2 group is 89.4° in 5NO2 and 32.4° in 8NO2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The isopropenyl derivatives of representative classes of polycyclic aromatic hydrocarbons (PAHs) having four and five fused‐ring systems, namely pyrene, chrysene, benzo[c]phenanthrene (BcPh), dibenzo[a,c]anthracene (benzo[f]tetraphene) and perylene, were synthesized by Wittig olefination from the corresponding acetyl‐PAHs. Under the influence of triflic acid (TfOH), the isopropenyl derivatives were converted to novel PAH dimers and/or phenalenes in a simple one‐pot procedure. A plausible mechanism for this process has been outlined, and the synthetic scope of this chemistry has been explored. Structural features in the PAH dimers were examined by DFT. As representative initial and final carbocation intermediates in the reaction sequence, stable carbocations derived from 3‐isopropenylperylene and from 4,6,6‐trimethyl‐6H‐dibenzo[a,kl]anthracene were generated and studied directly by NMR spectroscopy. The NMR characteristics and charge delocalization modes in the resulting benzylic carbocations are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)